Membrane-based technologies have the advantages of both low capital cost and high energy efficiency as compared to much older and established techniques such as cryogenic distillation, absorption, and adsorption. Membrane-based separation processes are widely adopted today in petrochemical, electronic, environmental, food, pharmaceutical, and biotechnology industries. For example, reverse osmosis (RO), has been successfully used for seawater desalination to meet freshwater demand in many regions of the world at low cost and minimum environmental impact. Other membrane-based filtration processes, such as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF), have similarly been successfully used in water treatment and purification. Furthermore, membrane distillation (MD) and pervaporation (PV) are emerging as new technologies for separations of greater difficulty. Finally, membrane-based selective gas separations are of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane-based selective gas separations have achieved commercial success, including nitrogen enrichment from air, carbon dioxide removal from hydrocarbons (e.g., from natural gas and enhanced oil recovery), and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams.
Polymeric membrane materials provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for liquid, vapor, or gas separations. For example, several polymeric membrane materials have been used in reverse osmosis desalination and water filtration, such as cellulose acetate, polysulfone coated with aromatic polyamides, poly(vinylidene fluoride), poly(acrylonitrile-vinyl chloride), etc. However, these polymeric materials have certain disadvantages and limitations. For example, cellulose acetate membranes are susceptible to microbiological attack and limited to a relatively narrow feed pH range. As an additional example, polyamide membranes have poor resistance to continual exposure to oxidizing agents such as chlorine (i.e., have low chlorine tolerance).
Fouling is another major problem in membrane applications. In general, fouling occurs either on the surface of a membrane or within its pores, and it causes a decrease in flux. Fouling is especially a challenge in reverse osmosis (RO) operations, as up to 10-15% of operational time may be spent on RO membrane cleaning. Furthermore, due to fouling, RO performance is lost over time, harsh cleaning shortens membrane life span, and increased operating cost is required to maintain productivity.
Other fouling controls in membrane operations include complex steps such as: (1) increasing hydrophilicity of membranes by grafting hydrophilic polymer chains on PVDF or polyamide; (2) incorporating silver/copper nanoparticles on the surface of the membranes; and/or (3) using electrically charged polymer nanocomposite membranes. Unfortunately, such control is often quite expensive and/or not long lasting. Furthermore, while fouling can be controlled to some extent by adding disinfectants, anti-scaling agents, and other pretreatment steps, such is merely a remedy, and does not present a permanent solution to fouling problems.